My U.S. Pat. No. 4,193,961 discloses a method of producing multifilament polypropylene yarn comprising heating polypropylene having a narrow molecular weight distribution with a swell value, or die swell, of less than 3 to a temperature at which it is molten, and extruding the molten polypropylene at a temperature less than 425.degree. F. into a plurality of filaments. The filaments are passed through a hot zone having a temperature sufficiently high to retard cooling of the filaments therein. The filaments are drawn down to their undrawn denier in the hot zone and then passed through a quenching zone in which cooling gas is directed over them. The combination of the swell value of the polypropylene, the temperature of extrusion, and the temperature of the hot zone interact to substantially eliminate the occurrence of resonance in the filaments as they are drawn down in the hot zone.
It was then discovered that when the extrusion temperature is dropped sufficiently below the region of 400.degree. F., unexpectedly uniform yarn can be produced with improved drawing characteristics. This is more fully described in my above-mentioned co-pending application Ser. No. 229,012 titled "Method of Extruding Polypropylene Yarn" filed Jan 27, 1981, which is hereby incorporated by reference.
With low extrusion temperatures, such as below 385.degree. F. and particularly below 365.degree. F., it was discovered that adiabatic conditions started occurring in the extruder as the rate of extrusion was increased. This could be somewhat controlled by using blowers to cool the metering and mixing zones of the extruder, but this is not a satisfactory long-term solution. It could also probably be controlled by increasing the depth of undercut in the metering zone of the extruder screw. However, it was desired to retain a shallow undercut in the metering zone to enhance mixing of the melt in the extruder when producing solution dyed yarns.